There are some industrial processes in which materials arise that can be thermally processed and hereby even regenerated if necessary. These include used foundry sands, for example, which can comprise residues of organic and inorganic binders, for example resins or bentonite. Bentonite is calcined by means of thermal processing, for example. Organic binders are oxidised, on the other hand, due to which the carbon content remaining in the sand can be reduced to less than 0.8 percent by weight.
With regard to the thermal processing of used foundry sands in rotary kilns, it is known in particular to heat these directly using a gas burner, in that the flame points into the drum chamber and the heating gases produced are conducted into the drum chamber, and also to operate in a direct flow. This means that the material to be processed is conveyed in the same direction through the kiln drum as the flow direction of the heating gases.
Flue gases arise in the thermal processing. In such an operation of a rotary kiln, however, these have a relatively high carbon monoxide (CO) concentration. The CO content can only be reduced by an afterburning system, which causes the operating costs of the system to increase.
Moreover, in direct flow operation the heating gas coming from the gas burner must have a relatively high temperature, especially in the starting area of the rotary kiln, as the regenerated used foundry sand must have a temperature of more than 700° C. on leaving the rotary kiln. Since the drum atmosphere cools down in the conveying direction or flow direction, an adequate starting temperature must be ensured accordingly at the start. However, this leads to high demands on the temperature resistance of the kiln material, especially in the starting area of the kiln drum, which likewise results in high costs.
It is true that there are basic approaches to operating a rotary kiln in counterflow, in which the conveying direction of the material is opposed to the flow direction of the heating gas and the material exits the kiln drum in the area of the burner flame. In this case, although the temperature at the burner flame can be lower than in direct flow operation, the exit temperature of the flue gases, which cool down while flowing through the kiln drum, then also drops. This can lead to an undesirable condensing of hydrocarbons in the flue gas conduits.
Even in the case of counterflow operation, afterburning of the flue gases is also necessary, as in addition to the vaporised hydrocarbons, dioxins can also arise from the organic binder, and these must be removed from the flue gases.
Similar problems arise in the thermal processing of road construction material. Older roads in particular were often constructed using a road surface in which pitch (coal tar) was used as a binding agent. However, pitch contains a high proportion of environmentally harmful polycyclic aromatic hydrocarbons (PAHs) with the key substance benzo(a)pyrene. The use or recycling of such road construction material is now no longer permitted.
The direct recycling of purely mechanically processed road construction material containing pitch is ruled out, therefore, and the grit contained in it must be freed from the binding agent containing pitch before the grit can then be used for the production of new asphalt.